Conventional automatic transmissions include a hydraulic control system that governs transmission operating pressure, fluid flow distribution for cooling, lubrication and other purposes as well as the actuation of various transmission components, e.g., clutch assemblies. Fluid is drawn by the pump from the fluid reservoir and delivered to the various transmission components via the control system.
A transmission pump is provided that derives its power from the engine crankshaft. In the case of transmissions with positive displacement pumps, flow rate of transmission fluid at the pump outlet is proportional to engine speed. As engine speed rises, a speed will be reached at which atmospheric pressure will not provide sufficient acceleration to the operating fluid to completely fill the pump rotating elements. This speed is known as the High Speed Fill Limit (HSFL), above which cavitation will occur in the pump, causing reduction in flow rate, unwanted wear on transmission components, pressure fluctuations in the hydraulic system and undesirable noise pollution.
Over some portion of their operating speed range, most positive displacement pumps provide more flow than is required by the transmission. In order to control transmission operating pressure, this excess flow is diverted via a primary pressure regulating valve. Historically, this excess flow was exhausted to the fluid reservoir. An improvement upon this practice is to recirculate the fluid to the pump inlet, thus returning some of the flow energy from the high pressure pump outlet to the pump inlet. More recently, some transmissions have included a jet pump in the recirculation path, maintaining a greater percentage of the flow energy and thereby elevating the pump's inlet pressure. An increased inlet pressure is desirable as it reduces the tendency for cavitation within the pump. This boosting feature is commonly accomplished by forming a nozzle within the recirculation path, integral to the passages formed within the structural components. While this provides the desired boost in the inlet pressure of the pump, the design is inflexible and cannot be altered without changing the structural components. For example, with a nozzle design integrated into a cast housing, altering the jet pump geometry requires expensive changes, such as casting die alteration or replacement. Additionally, the limitations associated with integrating the nozzle into a casting restrict the freedom in specifying the nozzle's geometry.
An alternate approach for incorporating a jet pump in the recirculation path is to position two components such that a nozzle-shaped passage is created between them. However, in such configurations the velocity of the jet stream is contingent upon those components fitting together in the precise manner in which they were designed. Accordingly, the stack up tolerances of the adjacent components must be taken into consideration in designing the jet feature of the pump. Due to these tolerances, the flow area defined by these components may be greater or less than the intended value, thus reducing the effectiveness of the design.
An alternate means of achieving a jet pump is desirable. It is desirable to have a transmission with a jet pump having a nozzle that is separable from the surrounding components. Additionally, it is desirable to achieve a jet pump that directs the jet stream through a center section of the nozzle so as to reduce reliance on the dimensions of other components.